A thorough understanding of the molecular mechanism of oxidative phosphorylation is of vital importance to health, since it is the backbone of our metabolic system which supports healthy growth and reproduction, the repair of damages, the defense against diseases as well as various intellectual and physical activities. Our previous studies showed that radioactive uncouplers with reactive side-chains can be synthesized and used to label in a highly specific manner the proteins which play an important role in the coupling of oxidation to phosphorylation in liver or heart mitochondria. These labeled proteins are being separated and studied by a combination of gel electrophoresis and isoelectric focussing techniques. The chemically modified mitochondria from both liver and heart are now being studied by chemical, kinetic, physiological and spectrometric methods.